The invention relates to silver halide photography. More specifically, the invention relates to radiation-sensitive high bromide emulsions prepared in the presence of starch peptizer and photographic elements employing such emulsions.
The most widely used forms of photographic elements are those that contain one or more silver halide emulsions. Silver halide emulsions are usually prepared by precipitating silver halide in the form of discrete grains (microcrystals) in an aqueous medium. An organic peptizer is incorporated in the aqueous medium to disperse the grains. Varied forms of hydrophilic colloids are known to be useful as peptizers, but the overwhelming majority of silver halide emulsions employ gelatino-peptizers. A summary of conventional peptizers, including gelatino-peptizers, is provided by Research Disclosure, Vol. 389, September 1996, Item 38957, II. Vehicles, vehicle extenders, vehicle-like addenda and vehicle related addenda, A. Gelatin and hydrophilic colloid peptizers. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. The term xe2x80x9cvehiclexe2x80x9d includes both the peptizer used to disperse silver halide grains as they are being formed and the binder used in coating emulsion and processing solution penetrable layers of photographic elements. Gelatin and gelatin derivatives are commonly employed to perform the functions of both peptizer and binder. While gelatin is by far the most widely used peptizer in the photographic emulsion arts, it has been shown that water dispersable starches may also be used as a peptizer to make silver halide emulsion grains (U.S. Pat. No. 5,284,744).
A dramatic increase in photographic speeds in silver halide photography began with the introduction of tabular grain emulsions into silver halide photographic products in 1982. A tabular grain is one which has two parallel major faces that are clearly larger than any other crystal face and which has an aspect ratio of at least 2. The term xe2x80x9caspect ratioxe2x80x9d is the ratio of the equivalent circular diameter (ECD) of the grain divided by its thickness (the distance separating the major faces). Tabular grain emulsions are those in which tabular grains account for greater than 50 percent of total grain projected area. Kofron et al U.S. Pat. No. 4,439,520 illustrates the first chemically and spectrally sensitized high aspect ratio (average aspect ratio  greater than 8) tabular grain emulsions. In their most commonly used form tabular grain emulsions contain tabular grains that have major faces lying in {111} crystal lattice planes and contain greater than 50 mole percent bromide, based on silver. A summary of tabular grain emulsions is contained in Research Disclosure, Item 38957, cited above, I. Emulsion grains and their preparation, B. Grain morphology, particularly sub-paragraphs (1) and (3).
Pushing starch made emulsions towards high speed has been hampered by difficulties in making large tabular grain sizes. The use of less grain-growth-restraining cationic starches as a peptizer for the precipitation of high bromide {111} tabular grain emulsions has addressed such difficulty, as taught by Maskasky U.S. Pat. Nos. 5,604,085, 5,620,840, 5,667,955, 5,691,131, and 5,733,718. Under comparable levels of chemical sensitization for high bromide {111} tabular grain emulsions, higher photographic speeds can be realized using cationic starch peptizers as taught by such patents. Alternatively, speeds equal to those obtained using gelatino-peptizers can be achieved at lower sensitization temperatures, thereby avoiding unwanted grain ripening. The use of oxidized cationic starches are particularly advantageous in exhibiting lower levels of viscosity than gelatino-peptizers, which facilitates mixing.
It has also been observed that employing a starch peptizer for emulsion grain precipitation may result in somewhat higher minimum densities (i.e., fog) than when a gelatino-peptizer is substituted, even when conventional antifoggants and stabilizers are present in the emulsion. It is likely a result of silver reduction by the starch aldehyde groups. This type of reduction is well known and is the basis for a test for aldehyde groups at ammonium hydroxide pH known as the Tollens"" test or xe2x80x9csilver mirrorxe2x80x9d test:
Rxe2x80x94CHO+2Ag(NH3)2++3OHxe2x88x92⇄2Ag+Rxe2x80x94COOxe2x88x92+4NH3+2H2O
Starch aldehyde groups can come about from three sources: (1) starch, being a polymer of glucose, a reducing sugar, has a natural aldehyde group at one end of each polymer strand, (2) hydrolysis of a polymer strand would make a new terminal aldehyde group in addition to the previous aldehyde group, and (3) partial oxidation of a Cxe2x80x94C bond in the glucopyranose ring can create two new aldehyde groups at the carbon bond scission point.
Fog may be reduced in starch precipitated emulsions by treating the emulsion (either during or after precipitation) with an oxidizing agent as disclosed, e.g., in U.S. Pat. Nos. 6,027,869 and 6,090,536, where the oxidizing agent establishes an oxidation potential capable of oxidizing metallic silver. Specifically preferred oxidizing agents employed during the preparation of high bromide emulsions precipitated with starch peptizers are halogens, e.g., bromine (Br2) or iodine (I2), and bromine or iodine generating agents. Elemental bromine and bromine-generating agents (such as an acidified solution of sodium hypochlorite containing sodium bromide) have been found to be particularly effective oxidants. When bromine or iodine is used as an oxidizing agent, the bromine or iodine is reduced to Brxe2x88x92 or Ixe2x88x92. These halide ions can simply remain with other excess halide ions in the dispersing medium of the emulsion or be incorporated within the high bromide grains without adversely influencing photographic performance.
The reaction of starch and oxidizing agents such as bromine at typically pH values conventionally used for gelatin peptized emulsions can also rapidly deplete the oxidizing agent, however, requiring the frequent addition of relatively high levels of oxidant to maintain desired high oxidation potentials sufficient for bleaching internal grain fog centers. Health concerns have arisen concerning the handling and generation of significant amounts of volatile halides during emulsion grain manufacture. Accordingly, it would be desirable to provide an emulsion grain precipitation process employing starch peptizer which would enable a reduction in the amount of volatile halides such as bromine which are added to or generated in an emulsion precipitation reaction vessel, and more preferably to completely eliminate the need to handle or add such volatile halides directly to the vessel, while still reducing fog generation in the precipitated emulsion grains.
In one aspect, this invention is directed to a process for precipitating a high bromide silver halide emulsion in an aqueous medium comprising precipitating high bromide radiation-sensitive silver halide grains in a reaction vessel in the presence of a peptizer comprising a water dispersable starch, wherein a strong oxidizing agent is added to the reaction vessel during or after the precipitation at a pH of less than 4.0 such that an oxidation potential of at least 650 mV (Ag/AgCl ref.) is achieved. An unexpectedly significant reduction in the rate of reaction between strong oxidants such as bromine and starch at such low pH has been found to enable an advantageous decrease in the amount of oxidant required to achieve and maintain a high oxidation potential during emulsion grain precipitation. Starch, unlike gelatin, also advantageously has adequate stability at the combination of high acidity and high emulsion precipitation temperatures.